Multi-fluid heat exchangers such as two or dual fluid heat exchangers have been known for a considerable period of time. One typical usage for such heat exchangers is in vehicular applications for cooling two different heat exchange fluids. However, they may also be used in other applications wherein one or both of the two fluids may be heated by a third fluid as well.
An early example of such a system intended for vehicular use is shown in U.S. Pat. No. 1,948,929 to Mac Pherson, issued Feb. 27, 1934. The Mac Pherson patent describes a dual fluid heat exchanger wherein part of the heat exchanger is used as a radiator to cool engine coolant while the remainder of the heat exchanger is utilized to cool a completely different type of fluid, namely, lubricating oil for an internal combustion engine.
A more recent example is found in the U.S. Pat. No. 6,394,176 to Marsais, granted May 28, 2002. In the Marsais patent, one fluid being cooled is a refrigerant for a vehicular air conditioning system while the other fluid is transmission lubricating oil.
In such systems, it is necessary to prevent intermingling of the two fluids being heated and/or cooled to prevent possible malfunction of the systems in which the systems are used. It is also highly desirable that such heat exchangers be of low volume and low weight, particularly when used in vehicular systems. Low volume minimizes spacial constraints on designers, allowing them to design aerodynamically “slippery” vehicles for enhanced fuel efficiency. Similarly, minimal weight also will improve fuel efficiency in a vehicle.
It is also highly desirable that such heat exchangers be inexpensive to fabricate and, in many instances, it is highly desirable to provide thermal isolation between that section of the heat exchanger dealing with one heat exchange fluid and the other section of the heat exchanger dealing with the second heat exchange fluid.
To achieve these goals, the above identified Marsais patent utilizes a heat exchanger employing tubular headers with a line tube receiving slots which in turn receive flattened tubes. Serpentine fins extend between adjacent ones of the tubes and are bonded thereto.
In one embodiment, to provide thermal isolation between the two sides of the heat exchanger, one of the tubes is a “dead tube” meaning that neither heat exchange fluid passes through it. In the headers, the dead tube is isolated, at each end, by two baffles which minimize heat transmission between the fluids in the headers. The present of the dead tube also minimizes heat transfer between the tubes via the fins customarily found in such heat exchangers.
One perceived difficulty in the fabrication of such a dual fluid heat exchanger resides in the possibility that if the heat exchanger is brazed during the fabrication process, the elevation of temperature may act upon gas within the dead tube and confined therein by the baffles, causing such gas to build up pressure and possibly cause minute leaks at the interface of one or more of the baffles and the corresponding header. If such leaks occur in baffles on opposite sides of the dead tube, the possibility of cross contamination exists. Moreover, there is no means by which any fluid that might leak into the space between the baffle can be readily discharged to avoid cross contamination.